Lithium-manganese spinels are presently considered useful positive electrode materials for 4 V secondary lithium and lithium-ion batteries. However, the stoichiometric spinel LiMn.sub.2 O.sub.4 exhibits poor cycling performance in comparison to other positive electrode materials used for 4 V batteries. Therefore, there have been numerous methods proposed in the art for increasing the cycling performance of LiMn.sub.2 O.sub.4.
For example, a portion of the manganese in the LiMn.sub.2 O.sub.4 spinel can be replaced with excess lithium as proposed in R. J. Gummow et al., Solid State Ionics, 69 (1994), p. 59; and U.S. Pat. No. 5,425,932 to Tarascon. Nevertheless, the stabilization of the LiMn.sub.2 O.sub.4 structure by doping the spinel with excess lithium to form Li.sub.1+X Mn.sub.2-X O.sub.4 is accompanied by a significant decrease in its specific capacity. This decrease is caused by the fact that each lithium ion in excess of the stoichiometric amount engages 3 Mn.sup.3+ ions by replacing one of them and changing the valence state of the other two from 3.sup.+ to 4.sup.+, thereby significantly reducing the number of manganese ions which can change their valence from 3.sup.+ to 4.sup.+ during the charge process.
Another proposed solution has been the replacement of a portion of the manganese ions with another cation as described, e.g., in U.S. Pat. No. 5,169,736 to Bittihn et al.; U.S. Pat. No. 5,478,674 to Miyasaka; EP 0744381; DE 4,435,117; GB 2,270,195; U.S. Pat. No. 5,677,087 to Amine et al.; and the Gummow et al. article, supra. Although the spinels substituted with cations other than lithium tend to show better capacity retention for the cathode material, there is still a substantial decrease in the specific capacity. This is typically due to the fact that the doping ion replaces a 3.sup.+ manganese ion, but cannot itself be transferred to 4.sup.+ during the charge process (e.g. Ni.sup.2+, Co.sup.3+, Cr.sup.3+ and Al.sup.3+), or it replaces a lithium ion in its tetrahedral site reducing the number of lithium ions which can be reversibly intercalated in the 4 V range (e.g. Fe.sup.3+, Ga.sup.3+, Ti.sup.4+ and V.sup.5+).
Other solutions have also been proposed. For example, U.S. Pat. No. 5,674,645 to Amatucci et al. proposes replacing a portion of the oxygen with other anions. Alternatively, U.S. Pat. No. 5,429,890 to Pynenberg et al. and U.S. Pat. No. 5,478,675 to Nagaura have proposed a composite physical mixture of LiMn.sub.2 O.sub.4 with other metal oxides. However, these methods have not provided the cycleability, specific capacity and structural stability desired in the art.